This invention relates generally to magnetic flowmeters, and more particularly to a magnetic flowmeter whose electromagnet is excited by a low-frequency wave whose frequency is less than the usual 50 or 60 Hz frequency of a commercial electric power line.
In a conventional magnetic flowmeter arrangement, the usual practice is to make use of a commercial AC power line as an excitation source in that high power can easily be obtained therefrom. However, AC excitation at the frequency of a commercial power line gives rise to eddy current problems. These eddy currents are induced by AC excitation and flow through the fluid to be metered, thereby introducing zero drift error.
To overcome this drawback, it has heretofore been proposed to make use of an excitation technique in which the source is a low-frequency wave whose frequency is less than the usual 50 or 60 Hz commercial power line frequency. In this known technique, a low-frequency wave is generated by a low-frequency sinusoidal oscillator energized by DC power. The output of this oscillator is amplified by a power amplifier and then applied to the excitation coils of the magnetic flowmeter.
This known technique has certain practical disadvantages. Thus in the case of a magnetic flowmeter having a relatively large diameter, because its power requirements are high, this dictates an installation including a large-sized DC source, a large-sized amplifier and so on. It is therefore not feasible to use this known technique in conjunction with a magnetic flowmeter having a relatively large diameter.
Another prior art technique makes use of a low-frequency rectangular wave that is produced by switching a constant current. However, this method has the following drawbacks:
1. To produce a constant current, a relatively complex power source is required.
2. Since a rectangular wave is used as an excitation current, its high-frequency harmonic components introduce inductive noise components in a flow signal. Thus one must employ a complicated circuit to eliminate these unwanted electroinductive noise components.
Yet another known excitation method for a magnetic flowmeter is that disclosed in the Mannherz et al. U.S. Pat. No. 3,783,687, wherein the output voltage of a full-wave rectifier is applied to the electromagnet through a switching element, thereby reversing the voltage polarity at a low-frequency rate. However, since the excitation wave has a rectangular wave form, this method also gives rise to unwanted harmonic noise components.